The structure of a GaN HEMT having an AlN (aluminum nitride) spacer will be described, taking an example of Non-Patent Document 1. In FIG. 1 of Non-Patent Document 1, a sectional view of the structure of the GaN HEMT having the AlN spacer is published. In this conventional structure, an AlN spacer layer (AlN) is crystal-grown on the whole surface of a channel layer (GaN buffer), a barrier layer (AlGaN) is crystal-grown on the AlN spacer layer, and a nitride film (Si3N4), and electrodes (a source electrode (Source), a gate electrode (Gate), and a drain electrode (Drain)) are formed on the barrier layer. The feature of this conventional structure is that the AlN spacer is simply crystal-grown on the whole surface.
The GaN HEMT is used in a high-output and high-frequency amplifier or a power switch circuit.
In order to improve the efficiency of an amplifier or a switch circuit, it is necessary to reduce access resistance that is present in series between a source electrode and a drain electrode. As a method for reducing this access resistance, a method for inserting an AlN spacer between AlGaN (aluminum gallium nitride) and GaN is employed. The polarization of AlN is larger than that of AlGaN, and therefore two-dimensional electron gas concentration is increased by inserting the AlN spacer, whereby the access resistance can be reduced.
FIG. 1 is a schematic sectional view showing an example of a structure of a conventional high electron mobility transistor having a nitride semiconductor used therein (GaN HEMT). As shown in FIG. 1, this conventional GaN HEMT is configured by inserting an AlN spacer, and includes a substrate 1, a buffer layer 2, a channel layer 3, a spacer layer 4, a barrier layer 5, an insulating film layer 6, a source electrode 7, a drain electrode 8, and a gate electrode 9. In this conventional structure, the AlN spacer layer 4 is present on the whole surface including the vicinity directly below the gate electrode 9.